Enhancement of high harmonic generation in liquid water by resonant excitation in the mid-infrared

Abstract We study high harmonic generation (HHG) in the visible spectral range generated in a flat liquid jet of H 2 O, excited by intense mid-infrared (MIR) radiation around 3 μm, which is nearly resonant with the OH vibrational modes. By introducing a weak excitation pulse prior to the intense MIR driver pulse for HHG, we observed an enhancement of the 5th, 7th, and 9th harmonics occurring approximately 2 ps after excitation and persisting for more than 120 ps, which is completely absent in the case of non-resonant D 2 O. These results suggest that the enhancement is caused by ultrafast heating through vibrational excitation.